Corrosion inhibitor composition applicable for aluminum and steel protection and procedure

ABSTRACT

A corrosion-inhibiting composition for application to a metal substrate, such as aluminum or steel, and in connection with a paint, and the synthesis of the composition is disclosed. The base of the inhibitor can be selected from the group consisting of 2,5-dimercapto-1,3,4 thiadiazole (DMTD), 2,4-dimercapto-s-triazolo-[4,3-b]-1,3-4-thiadiazole, trithiocyanuric acid (TMT), and derivatives of DMTD and TMT, including various N—,S— and N,N—, S,S— and N,S— substituted derivatives of DMTD, various S-substituted derivatives of Trithiocyanuric acid, 5,5 dithio-bis (1,3,4 thiadiazole-2(3H)-thione or (DMTD) 2  or (DMTD) n  the polymer of DMTD, dimer and polymers of TMT, Salts of DMTD of the general formula: M(DMTD) n  where n=1,2 or 3, and M is a metal cation and preferable M=Zn(II), Bi(III), Co(II), Ni(II), Cd(II), Pb(II), Ag(I), Sb(III), or Cu(II) (examples: ZnDMTD, Zn(DMTD) 2 , Bi(DMTD) 3 ) , similar salts of TMT , as for example, ZnTMT, in a ratio of 1:1, salts of (DMTD) n  of general formula M[(DMTD) n ] m , where n=2 or n&gt;2, m=1,2, or 3 and M is as above specified. Typical examples are: Zn[(DMTD) 2 ], Zn[(DMTD) 2 ] 2 , aryl or alkyl ammonium salts of DMTD or (DMTD) n , similar salts of TMT, quaternary ammonium salts of DMTD or (DMTD) n , and TMT, poly-ammonium salt of DMTD or (DMTD) n  and TMT formed with polyamines; inherently conductive polyaniline doped with DMTD or (DMTD) 2  and TMT ; inherently conductive polypyrrol and/or polythiophen doped with DMTD, (DMTD) 2  and/or TMT, micro or nano composites of poly DMTD/ polyaniline, poly DMTD/polypyrrol ,and poly DMTD/polythiophen, similar micro or nano composites with TMT DMTD or salts of DMTD or derivatives of DMTD and of TMT, as constituents of various pigment grade inorganic matrixes or physical mixtures, DMTD or salts of DMTD or derivatives of DMTD and TMT in encapsulated forms, such as: inclusions in various polymer matrices, or as cyclodextrin-inclusion compounds or microencapsulated form.

RELATED APPLICATION

[0001] This application claims the benefit of provisional application Serial No. 60/288,895 filed May 4 2001.

BACKGROUND OF THE INVENTION

[0002] Protection of aluminum against atmospheric tcorrosion constitutes a challenge of significant economic importance. Several distinct aluminum alloys are known, characterized by different susceptibility to atmospheric corrosion. Among others, aluminum alloys containing a small percentage of Cu are well known and valued for their excellent mechanical properties, as, for example, Al 2024 T-3, widely applied in aircraft manufacturing industry.

[0003] It is well known however, that due to copper rich intermetallic species present in the aluminum matrix, Al 2024 T-3 is also more susceptible to atmospheric corrosion. There are two distinct corrosion control technologies commonly applied to protect aluminum alloys (such as Al 2024 T-3) against atmospheric corrosion: conversion coatings and organic coatings.

[0004] As for conversion coatings, Alodine 1200 is one of the well-known corrosion inhibitor techniques widely applied for Al 2024 T-3 protection. It is based on soluble chromates containing CrO₄ ⁻⁻ as an inhibitor species and yields a robust conversion coating on aluminum substrates. A measure of its robustness, Alodine 1200 conversion coating on Al 2024 T-3 aluminum panels is known to resist salt spray exposure in excess of 300 hours, without pitting. In addition, conversion coatings are believed to enhance the adhesion of organic primers subsequently applied on aluminum substrates, a requirement also satisfied by Alodine 1200. Such procedures using chromates are thus considered to be the standard of the industry with respect to obtainable protection performance.

[0005] Aircraft primers and coil primers are the typical high performance organic coatings that are applied for protection of aluminum, especially in the aircraft manufacturing industry. A thickness of less than 20 micron is characteristic of these primers, which thus provide a negligible barrier function and, consequently, mandate the use of effective corrosion inhibitor pigments.

[0006] As is well known, pigment grade corrosion inhibitors used in organic primers must contain anionic species with inhibitor activity and must be characterized by limited, but effective, solubility in water. For these reasons, it will be apparent that Cro₄ ⁻⁻ is the corrosion inhibitor species preferred in both corrosion control technologies applied on aluminum for protection against atmospheric corrosion that is in conversion coatings and high performance organic primers.

[0007] SrCrO₄ is the corrosion inhibitor pigment of choice for aircraft and coil primers, and is the standard in the industry. Due to environmental concerns, finding a replacement for chromates in conversion coatings and organic coatings constitutes the objective of contemporary research in this field.

[0008] It is generally known, that the number of inorganic corrosion inhibitor species available for chromate replacement is limited essentially to a few, and specifically to MoO₄ ⁻⁻, PO₄ ⁻⁻⁻, BO₂ ⁻, SiO₄ ⁻⁻ and NCN⁻. As a consequence, all commercial non-chromate corrosion inhibitor pigments are molybdates, phosphates, borates, silicates or cyanamides, or combinations of these compounds.

[0009] In comparison to CrO₄ ⁻⁻, inherent limitations of their corrosion preventing mechanism render these above-specified species less effective inhibitors of corrosion, in general, and specifically of atmospheric corrosion of aluminum. Consequently, it appears that inorganic chemistry is unable to produce inhibitors of atmospheric corrosion of aluminum, which would be comparably effective, non-toxic alternative of CrO₄ ⁻⁻. In contrast, a large arsenal of organic corrosion inhibitor is known and applied in various corrosion control technologies. Excessive solubility in water and/or volatility of most of the known organic inhibitors appear to be the physical properties inconsistent with applications in conversion coating technologies and in organic coatings. As of up to date, no organic corrosion inhibitor is known to be an effective replacement of chromates in conversion coatings or organic coatings intended for aluminum protection.

SUMMARY OF THE INVENTION

[0010] It has been discovered pursuant to the present invention that organic compounds possessing cyclic structural features of aromatic character, carbocyclic and, specifically, heterocyclic aromatic structures containing one or multiple hetero species, such as, specifically, N, S, O atoms or combinations of the same, and preferably multiple —SH (mercapto) and=S, or thiol-thion functionalities attached, are effective inhibitors of corrosion of aluminum and its alloys. This discovery was not anticipated, considering that thiol-organic compounds (or/and H₂S) do not form essentially insoluble compounds (salts) with Al (III); as known, forming essentially insoluble (in water) compounds with ionic species of a specific metal is a general prerequisite for corrosion inhibitor activity of organic compounds on the respective metal substrate.

[0011] Specifically, the classification including di-mercapto and poly-mercapto compounds and their derivatives have been established as effective corrosion inhibiting compounds.

[0012] The following di- or poly-mercapto organic compounds are applicable:

[0013] di-mercapto derivatives of thiophene, pyrrole, furane, and of diazoles and thiadiazoles;

[0014] di- and tri-mercapto derivatives of pyridine, diazines, triazines and of benzimidazole and benzthiazole;

[0015] The following compounds and related derivatives are specifically identified

[0016] 2,5-dimercapto-1,3,4-thiadiazole and 2,4-dimercapto-s-triazolo-[4,3-b]-1,3-4-thiadiazole

[0017] 1,3,5-triazine-2,4,6(1H,3H,5H)-trithione, or trithiocyanuric acid, and dithiocyanuric acid,

[0018] dimercaptopyridine, 2,4-dithiohydantoine, and 2,4-dimercapto-6-amino-5-triazine.

[0019] Applicable derivatives of the above-specified di- and poly-mercapto organic compounds include:

[0020] salts formed with metal cationic species,

[0021] alkyl-, aryl-and quaternary-ammonium salts,

[0022] various N— and S-substituted derivatives, and

[0023] various N,N—, S,S— and N,S-substituted derivatives of the above compounds;

[0024] dimer and polymer derivatives of the above, resulted form oxidative dimerization or polymerization of di- and poly-mercapto compounds.

[0025] More specifically, it has been discovered that 2,5-dimercapto-1,3,4 thiadiazole symbolized by HS—CN₂SC—SH or “DMTD” and its derivatives inhibit atmospheric corrosion of aluminum, including Al 2024 T-3. It has been also proven that DMTD and various of its derivatives in pigment grade form are applicable as components of organic primers or in soluble or partially soluble form as an inhibitor constituent of conversion coating compositions intended for aluminum protection.

[0026] This discovery was not expected, considering that DMTD does not form essentially insoluble compounds with Al(III), of which this characteristic is generally a prerequisite for corrosion inhibition activity of organic compounds on metal substrates.

[0027] Along with DMTD, it has also been discovered pursuant to the present invention, that trithiocyanuric acid, or TMT, which can be classified as a tri-mercapto derivative, and its derivatives are also effective corrosion inhibitors of aluminum in a similar fashion as DMTD, and it has also been discovered that TMT and its derivatives are effective corrosion inhibitors when applied to galvanized steel and similar substrates.

BRIEF DESCRIPTION OF THE DRAWING

[0028] FIGS. 1-8 are graphical prints representing IR spectra of products produced pursuant to the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0029] The following description will describe in detail the synthesis of selected derivatives of 2,5-dimercapto-1,3,4 thiadiazole symbolized by HS—CN₂SC—SH or “DMTD”, and of selected derivatives of trythiocyanuric acid, or “TMT”, preferably used for application in a corrosion inhibitor in connection with a paint. DMTD, which is a di-mercapto derivative, and TMT, which is a tri-mercapto derivative, generally may be classified together. While it is believed that the corrosion inhibitor is applicable to a wide range of substrates, the following description reveals examples of applications to aluminum and galvanized steel.

[0030] The following are examples of DMTD, TMT, and derivatives of DMTD and TMT applicable to the practice of the invention:

[0031] 1. 2,5-dimercapto-1,3,4 thiadiazole (DMTD), 2,4-dimercapto-s-triazolo-[4,3-b]-1,3-4-thiadiazole, and trithiocyanuric acid (TMT);

[0032] 2. Various N—,S— and N,N—, S,S— and N,S— substituted derivatives of DMTD ; various S-substituted derivatives of trithiocyanuric acid;

[0033] 3. 5,5 dithio-bis (1,3,4 thiadiazole-2(3H)-thione or (DMTD)₂ or (DMTD)_(n) the polymer of DMTD; dimer and polymers of TMT

[0034] 4. Salts of DMTD of the general formula: M(DMTD)_(n), where n=1,2 or 3, and M is a metal cation and preferable M=Zn(II), Bi(III), Co(II), Ni(II), Cd(II), Pb(II), Ag(I), Sb(III), or Cu(II) (examples: ZnDMTD, Zn(DMTD)₂, Bi(DMTD)₃), similar salts of TMT , as for example, ZnTMT, in a ratio of 1:1 ;

[0035] 5. Salts of (DMTD)_(n) of general formula M[(DMTD)_(n)]_(m), where n=2 or n>2, m=1,2, or 3 and M is as above specified in 4. Typical examples are: Zn[(DMTD)₂], Zn[(DMTD)₂]₂;

[0036] 6. Ammonium-, aryl-, or alkyl-ammonium salts of DMTD, (DMTD)_(n), or 2,4-dimercapto-s-triazolo-[4,3-b]-1,3-4-thiadiazole. Typical examples: Cyclohexyl amine: DMTD, in ratios of 1:1 and 2:1; Di-cyclohexyl amine: DMTD, in ratios of 1:1 and 2:1; Aniline: DMTD, in ratios of 1:1 and 2:1; similar salts of TMT , as for example Di-cyclohexyl amine: TMT, in a ratio of 1:1;

[0037] 7. Quaternary ammonium salts of DMTD or (DMTD)_(n), and TMT

[0038] 8. Poly-ammonium salt of DMTD or (DMTD)_(n) and TMT formed with polyamines;

[0039] 9. Inherently conductive polyaniline doped with DMTD or (DMTD)₂ and TMT ;

[0040] 10. Inherently conductive polypyrrol and/or polythiophen doped with DMTD, (DMTD)₂ and/or TMT;

[0041] 11. Micro or nano composites of poly DMTD/polyaniline, poly DMTD/polypyrrol, and poly DMTD/polythiophen; similar micro or nano composites with TMT;

[0042] 12. DMTD or salts of DMTD or derivatives of DMTD and of TMT, as constituents of various pigment grade inorganic matrixes or physical mixtures;

[0043] 13. DMTD or salts of DMTD or derivatives of DMTD and TMT in encapsulated forms, such as: inclusions in various polymer matrices, or as cyclodextrin-inclusion compounds or microencapsulated form;

[0044] 14. various combinations of all of the above.

[0045] Likewise, it is understood that the above list is not conclusive, and similar compounds and derivatives will yield similar results.

[0046] Pigment grade forms of DMTD include Zn(DMTD)₂ and Zn-DMTD (among other salts of the former) and combinations of the latter with inorganic products or corrosion inhibitor pigments, such as, for example, ZnNCN, zinc phosphate, ZnO, and amorphous SiO₂ or combinations of the compounds.

[0047] In regard of the synthesis of the Zn salts of DMTD, it has been discovered pursuant to the present invention, that the spontaneous reaction of ZnO and DMTD yields exclusively Zn(DMTD)₂, as follows:

ZnO+2HS—CN₂SC—SH=Zn(—S—CN₂SC—SH)₂+H₂O   1

[0048] Reaction 1 implies that, apparently, Zn-DMTD cannot be produced by simply adjusting the DMTD/ZnO stoichiometric ratio to 1:1.

[0049] Di-mercapto derivatives useful in the practice of the invention are those having a limited solubility in water, from about 0.01 and 1000 millimoles (mmole) per liter. The greatly preferred range of solubilities is 0.1 to 10 mmole/1.

EXAMPLES Example 1

[0050] This example is intended to disclose the synthesis of Zn(DMTD)₂ according to the above presented Reaction 1.

[0051] As known, DMTD forms two distinct Zn(II) salts, that is Zn-DMTD or the 1:1 salts, and Zn(DMTD)₂ or the 1:2 salts. Each compound can be conveniently prepared by double decomposition in an aqueous medium, using, in corresponding stoichiometrical ratio, soluble Zn(II) salts and soluble salts of DMTD, such as Na₂-DMTD and Na-DMTD, respectively. Intuitively, both salts are also expected to form by reacting ZnO and DMTD, in a 1:1 or 1:2 stoichiometrical ratio, respectively.

[0052] It has been discovered pursuant to the present invention, however, that by reacting ZnO and DMTD, only Zn(DMTD)₂ forms. It will be apparent, that Reaction 1 is convenient in that it does not yield by-products. In practice, the synthesis according to reaction 1 was carried out as follows:

[0053] 1 mol (81.4 g) of high grade ZnO, of 0.25 micron average particle size, was re-slurried in 300 ml water by intense agitation and by heating to 50-60° C., after which the same conditions were maintained for 1 (one) hour. Concurrently, an aqueous suspension was prepared by stirring, at ambient temperature, 2 moles of DMTD (from R. T. Vanderbilt Company,Inc.) in 2000 ml water.

[0054] Reaction 1 was realized by gradually transferring, in about 30 min., the aqueous suspension of DMTD into the intensively stirred suspension of ZnO and by maintaining the same conditions, at 50-60° C., for 2 (two) hours. Subsequently, the solid phase was isolated by filtration, dried at 100-105° C. to 0.5-2% moisture content and pulverized. Notably, the process water was integrally recyclable.

[0055] Relevant analytical data and IR spectrum are presented below, in Table 1 and FIG. 1, respectively. TABLE 1 Measured quality parameters Determined values appearance Yellow powder specific gravity 2.2 solubility in water, at 24° C. 0.4 g/l pH (saturated extract) 4.5-5.0 yield, g 355.0

Example 2

[0056] This example is intended to disclose one synthesis procedure applicable for incorporating DMTD into a complex solid matrix corresponding to the general composition of 45% Zn(DMTD)₂/32% Zn₃(PO₄)₂ 2H₂O/23% ZnO.

[0057] In practice, the synthesis was carried out as follows:

[0058] 6.33 moles (515.0 g) of high grade ZnO (0.25 micron average particle size), was re-slurried in 2000 ml water at 50-60° C. and intense agitation for 1 (one) hour. After that, 1.5 moles of H₃PO₄, as 50% solution, were introduced gradually into the ZnO slurry and the same conditions were continued for 30 minutes. Subsequently, an aqueous suspension of 2.5 moles of DMTD in 1500 ml water was introduced in about 30 minutes. The intensively stirred slurry was heated to 75-80° C. and the same conditions were maintained for 2 (two) hours. The solid phase was isolated by filtration, dried at 100-105° C. to 0.5-2% moisture content and pulverized.

[0059] Relevant analytical data are presented below, in Table 2. TABLE 2 Measured quality parameters Determined values appearance Light yellow powder specific gravity 2.7 solubility, at 24° C. 0.3 g/l pH (saturated extract) 5-6 oil abortion, lbs/100 lbs 33 yield, g 992

Example 3

[0060] Application of a DMTD derivative as a corrosion inhibitor pigment:

[0061] A pigment grade composite of 45% Zn(DMTD)₂/32% Zn₃(PO₄)₂·2H₂O/23% ZnO, synthesized according to Example 2, was tested on aluminum, comparatively to a double control: commercial strontium chromate (Control A), which is the “gold” standard of the industry for corrosion inhibitor pigments and a molybdate-based product (Control B) considered representative of commercially available non-chromate corrosion inhibitor pigments. The test was performed in a typical two component aircraft primer formulation, specifically recommended for aluminum protection.

[0062] The description of the different versions of this formulation, the Test primer and of the Control A and Control B primers, are presented below. TABLE 3 Trade Names & Parts by Weight Components of Suppliers of Control Formulations Components Test A B Epoxy Base/Part A Epoxy Resin Shell Epon 1001 163.0 163.0 163.0 CX75 (1) Solvents Glycol ether PM 148.0 148.0 148.0 MIBK 36.7 36.7 36.7 Fillers RCL-535 TiO2 (2) 20.6 20.6 20.6 Min-U-Sil 15 (3) 26.0 26.0 26.0 12-50 Talc (4) 49.3 49.3 49.0 Corrosion Inhibitor Pigments Zn(DMTD)₂ in See Example 2. 78.0 — — solid matrix composite (See Example 2) Strontium SrCrO4-176 (5) — 107.5 — Chromate MoO₄ ⁽²⁻⁾ based Commercial (6) — — 86.0 pigment. Total part A-weight 551.0 551.0 551.0 Volume, gallons 50.0 50.0 50.0 CATALYST/PART B Hardener HY-815 67.1 67.1 67.1 Polyamide (7) Solvents Toluene 59.1 59.1 59.1 Isopropanol 218.5 218.5 21.5 Total Part B-weight 344.7 344.7 344.7 Volume, Gallon 50.0 50.0 50.0

Example 4

[0063] This example demonstrates the efficiency of DMTD derivatives in organic coatings in a corrosion inhibitor pigment.

[0064] In order to comparatively assess the corrosion inhibitor activity of DMTD derivatives, the Test primer of Example 3 as well as Control A and Control B primer formulations were applied by wire-wound rod, on several, Alodine 1200 (MIL-C-5541) treated bare 2024 T-3 aluminum panels (from The Q-Panel Co.), at 0.6-0.8 mils dry film thickness, aged for 7 days at room temperature, scribed and subsequently subjected to salt spray exposure (according to ASTM B-117) for 2000 hours. Notably, the scribes were applied in the typical cross form, at an approximate width of 2 mm, and, in order to remove the Alodine 1200 conversion coating from the area, at an appropriate depth.

[0065] By visual examination of their physical state at the end of the test period, the coatings' corrosion inhibitor performance, considered directly proportional to the tested pigment components' corrosion inhibitive activity was qualified. The scribed area was especially examined and the absence or presence of corrosion products, respectively, was interpreted as display of, or absence of, the respective corrosion inhibitor pigment's “throw power”. It will be apparent that the “throw power” is the discriminative characteristic of effective corrosion inhibitor pigments. Test results are summarized in Table 4. TABLE 4 Qualification of Coating/inhibitor Performance “Throw Pigment Tested Field Scribe Area Power” Obse Test primer/Zn Intact Void of yes (DMTD)₂ in a solid corrosion matrix (See Example 2) products Control A/SrCrO₄ Intact Void of yes corrosion products Control B/MoO₄ ⁽²⁻⁾ Intact Filled with no based pigment corrosion products

[0066] Both Control coatings and the Test coating were found intact in the field at the end of the test 5 period and it was concluded that 2000 hours of salt spray exposure was not sufficiently discriminant. Similarly to Cro₄ ⁻⁻, DMTD displayed throw power, however, by maintaining the scribe area void of corrosion products, in a passive state for the duration of the salt spray exposure test. In the same conditions, MoO₄ ⁻⁻ did not show throw power. It was concluded that DMTD derivatives possess effective corrosion inhibitor activity on aluminum and are applicable in pigment grades in organic primers intended for such.

Example 5

[0067] Applicability of DMTD in soluble forms in conversion coatings for aluminum protection.

[0068] DMTD based conversion coating was applied on several 2024 T-3 aluminum (the Test and Control) panels according to the following protocol: de-greasing, rinsing, deoxidizing (I), rinsing, deoxidizing (II), rinsing, treatment with DMTD (only of the Test panels), drying, post treatment with Zr(IV)/K₂ZrF₆ solution, rinsing and drying. In practice, rinsing (performed in stirred water at ambient temperature for 1 minute) and all operations were carried out by immersion as follows:

[0069] The Test and Control panels were de-greased in an alkaline cleaner solution (containing 2% of each: Na₂CO₃ and Na₃PO₄)at 50° C. for 1 minute, followed by rinsing at normal temperature for 1 minute. Deoxidizing was performed in two phases. Phase (I) was carried out in 25% H₂SO₄ solution at 60° C. for 1 minute, followed by rinsing, and phase (II) was performed in 50% HNO₃ solution at normal temperature for 30 seconds, followed by subsequent rinsing. DMTD based conversion coating was applied (only on the Test panels) by immersion for 10 minutes in saturated DMTD solution at 60° C., under agitation and, without rinsing, by subsequent drying at about 100-110° C. for approximately 10 minutes. Both the Test and the Control panels (the latter without DMTD coating) were post-treated by immersion, for 10 minutes, in a solution containing 0.5% ZrNO₃+0.5% K₂ZrF₆, at 60° C. under agitation. The treatment was finalized by rinsing and drying the Test and Control panels at 110° C. for 10 minutes.

Example 6

[0070] In order to assess the quality of DMTD-based conversion coating on 2024 T-3 aluminum, the Test panels were tested for corrosion resistance (according to ASTM B-117) and paint adhesion (tape test), in comparison with the Control panels, as well as with Alodine 1200 treated 2024 T-3 aluminum panels, the latter being the standard of the industry. The test results are presented below. TABLE 6 Corrosion resistance Rating* after 336 Paint adhesion Tested panels hours salt spray: by tape test: Test 8, Pass some pitting Control 0  Fail Standard 8, Pass some pitting

[0071] As the presented data indicates, DMTD-based conversion coating on 2024 T-3 Aluminum, applied according to the present invention, possesses robust resistance to corrosion and good paint adhesion, similar to chromate-based Alodine 1200 conversion coatings.

[0072] It was concluded that the treated DMTD derivatives are applicable as corrosion inhibitors in conversion coating technologies intended for aluminum protection.

Example 7

[0073] Di-cyclohexyl mono-ammonium salt of trithiocyanuric acid was synthesized according to the following procedure :

[0074] mole of di-cyclohexylamine (from Aldrich Chemical), dissolved in 0.15 moles of H₂SO₄ solution of approximately 20%, was subsequently reacted by agitation with 0.1 mole of Na-trithiocyanurate (from Aldrich Chemical) dissolved in 100 ml water. After the pH was adjusted to 6.5-7.0 , the resulted slurry was filtered, washed to soluble salt free condition, dried at approximately 100° C. and the solid product subsequently pulverized.

[0075] Yield: 34 g, 95% of theoretical.

[0076] The relevant IR spectrum is presented in FIG. 2.

Example 8

[0077] Di-cyclohexyl mono-ammonium salt of DMTD was synthesized as follows :

[0078] 0.2 moles of DMTD (from R. T. Vanderbilt Company, Inc.), previously dissolved in 150 ml aqueous solution containing 0.28 moles of NaOH, was reacted with 0.2 moles of di-cyclohexylamine dissolved in 100 ml solution containing 0.14 moles of H₂SO₄.

[0079] After the pH was adjusted to 6.5-7.0 , the resulted slurry was filtered, washed to soluble salt free conditions, dried and subsequently pulverized.

[0080] Yield: 66 g, approximately 90% of theoretical.

[0081] Relevant IR spectrum is presented in FIG. 3.

Example 9

[0082] Bi-DMTD (1:3)salt, or Bi(DMTD)₃₁ was synthesized as follows :

[0083] Initially, (A) was prepared by dissolving 0.15 moles of Bi(No₃)·5 H₂O in 1000 ml aqueous solution containing 0.5 moles of HNO₃, and (B) was prepared by dissolving 0.46 moles of DMTD in 1000 ml solution containing 0.92 moles of NaOH .

[0084] Bi(DMTD)₃ was subsequently obtained by introducing (A) and (B) , at identical delivery rates and simultaneously , into 200 ml water under intense agitation. After the pH was adjusted to 3.0 , the obtained slurry was stirred for 1 hour, filtered, washed to soluble salt free condition, dried at 110° C. overnight and pulverized.

[0085] Yield: 98 g, approx. 99% of theoretical.

[0086] Relevant IR spectrum is presented in FIG. 4.

Example 10

[0087] Poly-aniline/Trithiocyanuric acid (2:1) microcomposite was prepared according to the following procedure:

[0088] Initially, an aqueous suspension of Trithiocyanuric acid was prepared by reacting 0.05 moles of trisodium salt of trithiocyanuric acid (or 2,4,6-Trimercapto-s-triazine trisodium salt) dissolved in 200 ml water, with 0.16 moles of H₂SO₄ under intense agitation. Subsequently, a previously prepared aqueous solution, containing 0.1 mole aniline and 0.22 moles of HCl in 200 ml water, was added to the above-described suspension. Finally, 23 g ammonium persulfate (as an aqueous solution) and 0.5 g of FeCl₃ was introduced into the reaction system, which was stirred overnight at room temperature. The resultant dark green slurry was filtered, washed to soluble salt-free conditions, dried at 70-100° C. and pulverized.

[0089] Yield: 17 g Relevant IR spectrum is presented in FIG. 5.

Example 11

[0090] Zn(II) salt of trithiocyanuric acid, ZnTMT 1:1, was produced according to the following procedure:

[0091] Solution (A), containing 0.1 mole of trisodium salt of trithyocyanuric acid in 500 ml water, and solution (B), containing 0.1 mole of Zn(NO₃)₂ and 0.1 mole of HNO₃ in 500 ml water, were introduced simultaneously and at identical delivery rates, into 200 ml of intensively stirred water at about 50° C. The pH of the obtained slurry was adjusted to about 5 and after 1 (one) hour, during which the reaction conditions were maintained the same, the solid phase was separated by filtration, washed to soluble salt-free conditions, dried at 110° C. overnight and subsequently pulverized.

[0092] Yield: 22 g, 89% of theoretical.

[0093] Pertinent IR spectrum is presented in FIG. 6.

[0094] While the invention may be used in connection with a paint, it may also be used in connection with other protective coatings. For example, sol-gel protective coatings, which are generally known in the art, are silane-based, applicable for aluminum protection, and are considered as replacement of chromate-based conversion coatings such as Alodine 1200. The following example shows a practical procedure for applying the current invention in connection with a typical sol-gel process.

Example 12

[0095] Several Al 2024 T-3 Aluminum panels were degreased, and also de-oxidized in identical fashion as described in Example 5, and subsequently air-dried.

[0096] Solution (A) was prepared by dissolving 0.02 moles of diethylenetriamine and 0.01 moles of DMTD, in 100 ml water.

[0097] Solution (B) was prepared by the addition of 0.02 moles of tetramethoxysilane and 0.06 moles of glycidoxypropyltrimethoxysilane into 200 ml water and by adjusting the pH of the solution to about 4-4.5 with acetic acid, under continuous stirring at normal temperature.

[0098] After approximately 1 (one) hour, during which the hydrolysis process of the silane precursors proceeded in Solution (B), solution (A) was introduced into it under continuous agitation.

[0099] Test panels were prepared by the application, after about 10 minutes of stirring, of the resulted emulsion of silane condensate onto above specified aluminum panels at a spread rate of approximately 0.2-0.3 ml per 100 cm² and air-dried.

[0100] Control panels were prepared in similar fashion, except that Solution (A) was void of DMTD.

Example 13

[0101] Pigment grade Sr-doped amorphous silica of SrSiO₃·11SiO₂·5·7H₂O composition, containing approximately 9.5% Sr species, was synthesized according to the following procedure:

[0102] Initially, solution A was prepared by reacting 0.51 mole of SrCO₃ and 3.5 moles of HNO₃ and dissolving the composition in 1300 ml of water. Solution B was prepared by dissolving 1.9 moles of sodium silicate of Na₂O(SiO₂)_(3.22) composition (from Hydrite Chemical Co., WI.), in 900 ml of water.

[0103] Solutions A and B were delivered simultaneously and with identical rates for approximately 1 (one) hour into 500 ml of intensively stirred water at 70-85° C. At the end, the pH was adjusted to 8-8.5 and the same conditions were maintained for an additional 2 (two) hours, after which the resultant solid phase was separated by filtration, washed to soluble salt-free conditions, dried at approximately 105° C. overnight, and pulverized.

[0104] Relevant analytical data and IR spectrum results are presented below in Table 13 and FIG. 7, respectively. TABLE 13 Measured Parameters Determined Values appearance White powder specific gravity 1.8-1.9 pH (saturated extract) 9.0-9.3 oil absorbtion, lbs/100 lbs 52-60 Sr, % (calculated) 9.5 H₂O, % (by ignition at 600° C.) 16.5 yield, g 471

Example 14

[0105] A pigment grade mixture of trithiocyanuric acid+Sr-doped Amorphous Silica of SrSiO₃·11SiO₂·5H₂O+1TMT (approximate composition) , containing about 8% Sr (calculated) and 17% TMT (calculated) , was produced as follows:

[0106] 100 g of trithiocyanuric acid, in powder form, was blended into 460 g of Sr-doped amorphous silica in dry granular form. The Sr-doped amorphous silica was synthesized and processed as shown in Example 13. The obtained mixture was subsequently pulverized to a fineness of about 6 on the Hegman scale.

[0107] Trithiocyanuric acid was obtained from an aqueous solution of tri-sodium-trithiocyanurate, by adjusting the solutions pH to about 3, filtering, washing, and drying the resultant solid phase.

[0108] Relevant analytical data and IR spectrum results are presented below in Table 14 and in FIG. 7, respectively. TABLE 14 Measured Parameters Determined Values appearance Light yellow powder specific gravity 1.7 pH (saturated extract) 6.9 oil absorbtion, lbs/100 lbs 75-85 Sr, % (calculated) 7.9 TMT % (calculated) 17 yield, g 560

Example 15

[0109] This example is intended to demonstrate the application of trithiocyanuric acid (“TMT”) as a corrosion inhibitor constituent of an amorphous silica+TMT pigment grade mixture in a typical coil coating formulation.

[0110] The pigment grade mixture of SrSiO₃·11SiO₂·5H₂O+1TMT composition was synthesized according to the process in Example 14, and was tested (See Test formulation, Table 15) on galvanized steel (from L. T. V. Steel Co.), in comparison with commercial strontium chromate (Control A formulation, Table 15), the “gold” standard of the industry for corrosion inhibitor pigments, and respectively, Sr-doped amorphous silica synthesized according to Example 13 (Control B formulation, Table 15).

[0111] The typical solvent-borne polyester coil primer formulation is specifically recommended for galvanized steel protection. Description of the test formulation, and control formulations A and B are presented below in Table 15. TABLE 15 Parts by Weight Trade Names & Control Components of Suppliers of Test Formulation Formulations Components Formulation A B Polyester Resin EPS 3302 (1) 536.0 536.0 536.0 Solvents Aromatic 150 118.0 118.0 118.0 Diacetone 73.5 73.5 73.5 Alcohol Fillers RCL-535 TiO₂ (2) 46.0 46.0 46.0 Aerosil R972 (3) 2.1 2.1 2.1 Catalyst Cycat 4040 (4) 7.6 7.6 7.6 Hardener Cymel 303 (4) 73.6 73.6 73.6 Corrosion Inhibitor Pigments Strontium SrCrO₄-176 (5) — 143.5 — Chromate Sr-doped As shown in — — 120.0 amorphous Example 13 silica Sr-doped silica + As shown in 150.0 — — TMT pigment Example 14 grade mixture Total Weight 1006.8 1000.3 976.8

Example 16

[0112] This example demonstrates the applicability of di-mercapto and tri-thio derivatives according to the present invention, as corrosion inhibitor additives in paint formulations. Specifically, the application of trithiocyanuric acid-di-cycloamine, in a salt of a 1:1 ratio, as an additive in a typical coil primer formulation, is disclosed.

[0113] The coil primer formulation prepared was identical to the test formulation described in Example 15 (See Table 15), except that the corrosion inhibitor constituent consisted of 120 parts by weight Sr-doped Amorphous Silica, prepared according to example 13, and 30 parts by weight of trithiocyanuric acid-di-cyclohexylamine, in a salt of a 1:1 ratio. This was introduced into the formulation to end up with 1006.8 parts by weight of paint and ground to 6.5-7.0 fineness on the Hegman. The trithiocyanuric acid-di-cyclohexylamine 1:1 salt was synthesized according to Example 7 of the present invention.

[0114] Consequently, the corrosion inhibitor constituent of the test formulation according to Example 16 consists of an ordinary physical mixture of the above two components. The results are shown in Table 17 (See Example 17).

Example 17

[0115] This Example demonstrates the efficiency of di-mercapto derivatives, in general, and of trithiocyanuric acid and its derivatives, in particular, as corrosion inhibitor pigments or additives in coil primer formulations and on typical coil substrates, such as galvanized steel. It will be, however, apparent to one skilled in the art that the concept of the present invention applies for primers intended for steel protection in general.

[0116] In order to comparatively assess the corrosion inhibitor activity of trithiocyanuric acid and its derivatives, the test primers of Examples 15 & 16, along with control formulations A & B from Example 15, were applied by wire-wound rod, on several galvanized steel panels (from L. T. V. Steel Co.), at 0.6-0.7 mil dry film thickness, aged for at least 2 (two) days at room temperature, scribed and subsequently subjected to salt spray exposure (according to ASTM B-117).

[0117] The scribes were applied in the typical cross form, and, in order to cut through the protective galvanic zinc coating from the area of the scribes, at appropriate depth. During salt spray exposure, the coatings' physical state was assessed periodically by visual examination. Scribe areas were observed for the absence or presence of corrosion products (white rust), and “field” areas were observed for the physical integrity of coatings and the presence of white rust.

[0118] Notably, the protective performance of the tested coatings was qualified by the service life of coatings, defined as the total hours of salt spray exposure that result in extensive corrosion along the scribes and considerable corrosion in the “field” areas. Service life of a coating is considered directly proportional to the related pigments' or additives' corrosion inhibitor performance, which is conveniently qualified by E_(i), the Inhibitor Efficiency Index, defined as:

E _(i)=100[(service life)_(TEST)−(service life)_(CONROL)]/(service life)_(CONTROL).

[0119] It is important to note, that the service life of control formulation A from Example 15, containing SrCrO₄ as a corrosion inhibitor pigment, was considered as the test control, or (service life)_(CONTROL). It will be apparent, that values of E_(i)>0 indicate comparatively better corrosion inhibitor performance than the control's (SrCrO₄'s) performance, whereas values of E_(i)<0 indicate a poorer corrosion inhibitor performance than that of the control. The test results are summarized below in table 17. TABLE 17 Inhibitor Pigment or Service life of Test additive/coating Coating (hours) E_(i) % 1. Trithiocyanuric acid-di- 3000 87 cyclohexykamine, 1:1 salt and Sr-doped amorphous silica mixture, as described by the test primer in table 16 (Ex. 16). 2. Trithiocyanuric acid + Sr- 2000 25 doped amorphous silica pigment grade mixture, as described by the test primer in table 15 (Ex. 15). 3. SrCrO₄, as described by 1600  0 control A in table 15 (Ex. 15) 4. Sr-doped amorphous silica, 1000 −37  as described by control A in table 15 (Ex. 15).

[0120] The disclosed E_(i) values indicate that, in comparison with Sr-doped amorphous silica, trithiocyanuric acid and trithiocyanuric acid-di-cyclohexylamine, 1:1 salt significantly extend the service life of the coatings. Trithiocyanuric acid extends the service life of coil coatings on galvanized steel by 100% over Sr-doped amorphous silica, and tithiocyanuric acid-di-cyclohexylamine, 1:1 salt, extends the service life by 200% over Sr-doped amorphous silica. Likewise, both compounds display considerably better corrosion inhibitor performance than SrCrO₄, and more specifically trithiocyanuric acid-di-cyclohexylamine, 1:1 salt displayed the best corrosion inhibiting performance. Also, Sr-doped amorphous silica, as expected, displayed significantly poorer inhibitor performance than SrCrO₄. 

What is claimed is:
 1. A process for protecting an aluminum substrate against atmospheric corrosion comprising: providing an aluminum surface to be protected, applying a coating to said surface of a protective material selected from a group consisting of di-mercapto and poly-mercapto compounds and their derivatives, said coating substance having a limited solubility in water of between about 0.01 and about 1000 mmoles/per liter of water.
 2. A process for protecting an aluminum substrate against atmospheric corrosion comprising: providing an aluminum surface to be protected, applying a coating to said surface of less than approximately 20 microns in thickness of a protective material selected from a group consisting of di-mercapto and poly-mercapto compounds and their derivatives, said coating substance having a limited solubility in water of between about 0.01 and about 1000 mmoles/per liter of water.
 3. A process according to claim 1 wherein said coating substance has a limited solubility in water of between about 0.1 and about 10 mmoles/per liter of water.
 4. A process according to claim 1 wherein said coating substance is applied as a saturated solution and is subsequently dried whereby a conversion coating is formed on said substrate, said conversion coating being subsequently coated with a paint.
 5. A process according to claim 1 wherein said coating substance is incorporated into a silane-based gel coating.
 6. A process according to claim 1 wherein saidsubstance is selected from the group consisting of 2,5-dimercapto-1,3,4 thiadiazole (DMTD), 2,4-dimercapto-s-triazolo-[4,3-b]-1,3-4-thiadiazole, trithiocyanuric acid (TMT), derivatives of DMTD and derivatives of TMT.
 7. A process according to claim 1 wherein said substance is selected from the group consisting of 2,5-dimercapto-1,2,4 thiadiazole; 2,4-dimercapto-s-triazolo-[4,3-b]-1,3-4-thiadiazole; 5,5 dithio-bis (1,3,4 thiadiazole-2(3H)-thione and (DMTD)₂; (DMTD)_(n); M(DMTD)_(n) where n=1,2 or 3, and M is a metal cation selected from the group consisting of Zn, Bi, Co, Ni, Cd, Pb, Ag, Sb, or Cu; aryl ammonium salts of DMTD; aryl ammonium salts of (DMTD)_(n); alkyl ammonium salts of (DMTD),; quaternary ammonium salts of DMTD; quaternary ammonium salts of (DMTD)_(n); polyaniline, polythiophen, and polypyrrol doped with DMTD; polyaniline, polythiophen, and polypyrrol doped with (DMTD)₂ and micro and nano composites of poly DMTD/polyaniline, DMTD polythiophen and poly DMTD/polypyrrol.
 8. A process according to claim 1 wherein said coating substance is applied by incorporating the same in a curable polymeric coating composition and applying said composition over said substrate.
 9. A process according to claim 1 wherein said coating substance is selected from the group consisting of: 2,5-dimercapto-1,3,4 thiadiazole (DMTD); 2,4-dimercapto-s-triazolo-[4,3-b]-1,3-4-thiadiazole; an organic derivative of DMTD; N—, S— and N, N—, S— and N, S-substituted derivatives of DMTD, 5,5 dithio-bis(1,3,4 thiadiazole-2(3H)-thione or (DMTD)₂ or (DMTD)_(n) a polymer of DMTD; a salt of DMTD of general formula, M(DMTD)_(n), where n=1,2 or 3, and M is a metal cation and M=Zn, Bi, Co, Ni, Cd, Pb, Ag, Sb, or Cu; a salt of (DMTD)_(n) of general formula M[(DMTD)_(n)]_(m), where n=2 or n>2, m=1,2, or 3 and M=Zn, Bi, Co, Ni, Cd, Pb, Ag, Sb, or Cu; aryl and alkyl ammonium salts of DMTD and (DMTD)_(n); quaternary ammonium salts of DMTD and(DMTD)_(n); polyammonium salts of DMTD and (DMTD)_(n) formed with a polyamines; polyaniline, polypyrrol and polythiophen doped with DMTD; polyaniline, polypyrrol and polythiophen doped with (DMTD)₂; a micro and nano composites of poly DMTD/polyaniline, poly DMTD/polypyrrol, and poly DMTD/polythiophen; DMTD , salts of DMTD , and derivatives of DMTD, as constituents of an inorganic matrix; DMTD , salts of DMTD ,and derivatives of DMTD encapsulated form in a polymer matrix, or as a cyclodextrin-inclusion compound; and a combination of said forms.
 10. A process according to claim 1 wherein said coating substance is selected from the group consisting of: Trithiocyanuric acid (TMT); S-substituted derivatives of TMT; a salt of TMT of general formula, M(TMT)_(n), where n=1,2 or 3, and M is a metal cation and M=Zn, Bi, Co, Ni, Cd, Pb, Ag, Sb, or Cu; aryl and alkyl ammonium salts of TMT; quaternary ammonium salts of TMT; polyamines formed with TMT; polyaniline doped with TMT; polypyrroline and polythiophen doped with TMT; micro and nano composites of poly TMT/polyaniline, poly TMT/polypyrrolidone, and poly TMT/polythiophen; TMT, salts of TMT, and derivatives of TMT, as constituents of an inorganic matrix; salts of DMTD ,and derivatives of TMT encapsulated form in a polymer matrix, or as a cyclodextrin-inclusion compound; and a combination of said forms.
 11. The coating substance of claim 1 wherein the coating substance is used in a paint.
 12. The paint according to claim 9, whereby the paint is applied to an aluminum surface.
 13. A process for protecting a steel substrate against atmospheric corrosion comprising: providing a steel surface to be protected, applying a coating to said surface of less than approximately 20 microns in thickness of a protective material selected from TMT and TMT derivatives.
 14. A process according to claim 11 wherein said coating substance is applied as a saturated solution and is subsequently dried whereby a conversion coating is formed on said substrate, said coating being subsequently coated with a paint.
 15. A process according to claim 11 wherein said coating substance is incorporated into a silane-based gel coating.
 16. A process according to claim 11 wherein said coating substance is selected from the group consisting of: Trithiocyanuric acid (TMT); S-substituted derivatives of TMT; a salt of TMT of general formula, M(TMT)_(n), where n=1,2 or 3, and M is a metal cation and M=Zn, Bi, Co, Ni, Cd, Pb, Ag, Sb, or Cu; aryl and alkyl ammonium salts of TMT; quaternary ammonium salts of TMT; polyamines formed with TMT; polyaniline doped with TMT; polypyrroline and polythiophen doped with TMT; micro and nano composites of poly TMT/polyaniline, poly TMT/polypyrrolidone, and poly TMT/polythiophen; TMT, salts of TMT, and derivatives of TMT, as constituents of an inorganic matrix; salts of DMTD ,and derivatives of TMT encapsulated form in a polymer matrix, or as a cyclodextrin-inclusion compound; and a combination of said forms.
 17. The steel substrate of claim 11, wherein said steel substrate comprises a galvanized steel.
 18. The coating substance of claim 11, wherein the coating substance is used in a paint.
 19. The paint according to claim 14, whereby the paint is applied to a steel surface.
 20. The paint according to claim 14, whereby said paint is applied to a galvanized steel surface. 